Make sure the plug is grounding out when you pull the motor over. Do this by ensuring that the plug is connected to the coil, then hold a rubber part of the coil wire (or spark plug boot), and hold the plug against a head-bolt (you could also rest the plug on a head-bolt without holding it). Some other things you could do include rubbing the flywheel magnet with a piece of sandpaper or a wire brush to get whatever rust my be on there, off. Also, as suggested above, clean the underside of the coil (may be rusty). If you haven't already, remove the flywheel and the points and condenser cover and clean and re-gap the condenser. Also make sure that the little plunger like thing is moving up and down smoothly. Do this by rotating the crankshaft and keeping an eye on the points and condenser. If the small spring loaded arm moves up and down the plunger is free and working. Make sure to set the correct gap of everything when putting it back together. This is super simple if you have a "feeler gauge set" and a standard business card.
For memory the gap spec. for between the condenser and the arm is .010. Gapping the condenser is fiddly and may take some trial and error.
To gap the coil, simply loosen the coil bolts, place a business card underneath the coil, hold the card and rotate the flywheel until the magnets are underneath the coil. The coil should sit down automatically. Once it does, tighten the bolts and remove the card. May take a few goes to get it right.
Not sure if you already know this stuff, it's just good stuff to know and try if you have not already.
-Jack